1. Field of the Invention
This invention relates to ferroelectric liquid crystal display devices, specifically a device with improved contrast ratios.
2. Discussion of Prior Art
Liquid crystal display devices are well known. They typically comprise a liquid crystal cell formed by a thin layer of a liquid crystal material held between two glass walls. These walls carry transparent electrodes which apply an electric field across the liquid crystal layer to cause a rotation of the molecules of liquid crystal material. The liquid crystal molecules in many displays adopt one of two states of molecular arrangement, one of which may be a voltage OFF state and the other a voltage ON state. Information is displayed by areas of liquid crystal material in one state contrasting with areas in the other state. One known display is formed as a matrix of pixels or display elements produced at the intersections between column electrodes on one walls and row electrodes on the other wall. The display is addressed in a multiplex manner by applying voltages to successive row and column electrodes. Another type of display is a shutter which is addressed in a direct drive manner.
Liquid crystal materials are of three basis types, nematic, cholesteric, and smectic each having a distinctive molecular arrangement.
The present invention concerns ferro electric smectic liquid crystal materials. Devices using this material form the surface stabilised ferroelectric liquid crystal (FLC) device. These devices can show bistability, ie the liquid crystal molecules, more correctly the molecular director, adopt one of two alignment states on switching by positive and negative voltage pulses and remain in the switched state after removal of the voltage. This behaviour depends upon the surface alignment properties. Some types of surface alignment will produce a device in which the switched states remain after removal of the voltage, other types of surface alignment will produce a device in which the states may randomly decay on removal of the voltage. The switched states may be stabilised by the presence of an ac bias. The actual states achieved may be dependent upon the amplitude of any ac bias present. The ac bias may be provided by the data (column) voltages in a mulitplexed device. This property, together with the fast switching speed, makes FLC devices suitable for large displays with a large number of pixels or display elements. Such ferroelectric displays are described for example in: N A Clark and S T Lagerwall, Applied Physics Letters Vol 36, No 11 pp 889-901, June, 1980; GB-2,166,256-A; U.S. Pat. No. 4,367,924; U.S. Pat. No. 4,563.059; patent GB-2,209,610 [Bradshaw end Raynes]; R B Meyer et al, J Phys Lett 36, L69, 1975.
The two switched states are made visible by arranging the liquid crystal cell between two crossed polarisers. One of the polarisers is aligned approximately parallel to one of the switched directions. This is known for example from GB 2,209,610.
To improve device legibility and device usefulness all device designers seek to obtain maximum contrast between the two switched states. The present invention improves this contrast of existing FLC devices.